Imported PLY file appears blank
I think most .PLY object come very big. 
The object is here but you need to scale it down a lot and center it.
This is my process with this sample file :
http://people.sc.fsu.edu/~jburkardt/data/ply/airplane.ply


file import .ply
CTRL-ALT-SHIFT-C to set the Origin to Geometry or in the menu 

ALT-G to make sure your object is in the center
scale S to something like 0.02
I would suggest to immediately Apply Scale (CTRL-A) to ensure correct scaling.